A laser system may be classified as a semiconductor laser, a solid-state laser, a gas laser, and a liquid laser according to a gain-medium used therefor, and a laser system essentially comprises a pumping system, a gain-medium and a resonator. When we want to get optical pulses in a laser system, a Q-switch is usually employed. The Q-switch is an active Q-switch or a passive Q-switch, wherein a saturable absorber can be used as the passive Q-switch resulting in a low cavity Q during the initial pumping of the gain medium. When the gain is sufficient to overcome the low intensity losses and the lasing begins, the saturable absorber is quickly “bleached” to a low-loss state and a giant pulse is generated.
Diode-pumped passively Q-switched solid-state lasers using saturable absorbers have attracted significant attention because of their compactness and simplicity in operation. Numerous saturable absorbers have been developed to replace the dyes used in solid-state lasers, such as Cr4+-doped crystals and semiconductor saturable absorber mirrors (SESAMs). Nowadays, Cr4+:YAG crystals are no doubt the most commonly used saturable absorbers, but it is quite inconvenient to apply Cr4+:YAG crystals as saturable absorbers in conventional Nd-doped vanadate crystals because the absorption cross sections of Cr4+:YAG crystals are not large enough for a good Q-switched criterion.
InGaAs/GaAs quantum wells (QWs) have been utilized as SESAMs; however, the lattice mismatch thereof leads to a limitation of a modulation depth that is defined as a maximum absorption change between low and high intensities. As a consequence, output pulse energies and conversion efficiencies with InGaAs SESAMs are generally significantly lower than those with Cr4+:YAG crystals.
Therefore, it is highly desirable to develop a saturable absorber with a large absorption cross section, a large modulation depth, and a high damage threshold. In order to overcome the mentioned drawbacks in the prior art, a semiconductor saturable absorber and the fabrication method thereof are provided in the present invention.